The interface between the cell cycle and plant growth regulators: a mini review

The aim was to review knowledge about the interface between plant growth regulators and molecular checkpoints of the cell cycle. At what level of biochemical regulation of the cell cycle do plant growth regulators interface ? Are there different levels of interfacing dependent on the plant growth regulator involved ? As a preamble to tackling these questions, we overview the eukaryotic cell cycle with particular emphasis on checkpoints that regulate the transition from G0-G1-S-phase and G2-M. Cytokinins feature strongly as activators of cell division in plants both in vivo and in vitro. Recent research has shown that zeatin treatment led to the up-regulation of CycD3 in Arabidopsis. This is a D-type cyclin showing strong homology with vertebrate D cyclins which themselves are up-regulated by extracellular growth factors. Benzyladenine treatment can also shorten the duration of S-phase through recruitment of latent origins of DNA replication. Kinetin is involved in the phosphoregulation of the G2-M checkpoint; the major cyclin-dependent kinase (Cdk) at this checkpoint has recently been shown to be dephosphorylated as a result of cytokinin treatment, an effect which can also be mimicked by the fission yeast Cdc25 phosphatase. Hence, a picture emerges of a cytokinin-induced continuum of cell cycle activation through the up-regulation of a plant D-type cyclin at the G1 checkpoint and the phosphoregulation of the Cdk at the G2/M checkpoint. During S-phase, we argue for a link between cytokinins and the proteins associated with replication origins. Gibberellic acid (GA) treatment induces internode elongation. In deepwater rice, this response is mediated, at least partly, by a GA-induced up-regulation of a cyclin-Cdk at the G2-M checkpoint. Recent evidence has also linked abscisic acid to a cyclin-dependent kinase inhibitor. These, so-called CKIs are negative regulators of Cdks which fits with ABA's general role in growth inhibition; we await news of ethylene interactions. We highlight two instances of plant growth regulator-cell cycle interfacing during development, arguing for an involvement in microtubule orientation as a prerequisite to leaf initiation, and suggest a link between IAA and the activation of cell divisions in the pericycle required for lateral root initiation. A new D-type cyclin, recently discovered in Arabidopsis, may have a key role in this process. Finally, a model is presented which features a generalised cyclin-Cdk checkpoint exhibiting various interfaces with the plant growth regulators.